Generalise 'delete' action over type class

[frasertweedale]

Deleting the current item from a list is conceptually simple. In case other lists-with-deletable-items
emerge in purebred, we should abstract it over a type class, e.g.

class HasList n => HasEditableList n where               
  allowDeleteLast :: Proxy n -> Bool                     
deleteSelectedItem :: HasEditableList ctx => Action v ctx